Can I use the sapling addon in Blender render instead of Cycles
The sapling addon only creates geometry for your tree, not materials, and has such it is mostly render engine agnostic.

That means you can use it with any render engine you like, the addon only creates the shape for your tree and probably does some basic UV unwrapping for the leaves which can be used on any material.

Material creation is then up to you, and as no effect on the way the geometry was created. If you want to create materials for your tree you must search for adequate tutorials for Blender Internal material system.

If you want the transparent parts of our texture to affect your leaves you should tick the appropriate box in the texture settings to affect material transparency Under the Properties Window > Textures > Influence > Alpha